The demand for energy storage systems that are compact, lightweight, safe and powerful is skyrocketing with the worldwide proliferation of portable electronic devices, including notebook and tablet computers, PDAs, camcorders, digital cameras, mobile phones and increase electronics usage by the military.
Batteries provide a limited amount of energy and have shown slow improvement. Although Moore's Law has computing capability doubling in capacity every two-three years or so, increases in battery capacity have not kept pace. As a result, a “Power Gap” exists between energy demand and the energy available in today's rechargeable batteries. With current battery technology at risk of not keeping pace with product evolution and new technology adoption, military decision makers and consumer electronic manufacturers are turning to alternative energy sources to address the Power Gap, and provide clean renewable power sources which enable “always on” capability at significantly reduced weight. Fuel cells are one of the most promising technologies that can bridge the Power Gap and provide portable products with a significant increase in runtime and greater convenience of use.
One type of fuel cell is proton exchange membrane (PEM) fuel cells; their distinguishing features include lower temperature/pressure ranges (50 to 100° C.) and a special polymer electrolyte membrane. The fuel in a PEM fuel cell is typically hydrogen; in order to utilize hydrogen for a consumer electronic device fuel cell, it needs to be delivered or supplied as part of a solid material that can store it.
The key limiting factor in the widespread adoption of PEM fuel cell based power systems is hydrogen fuel storage. Realization of a viable hydrogen storage solution will have a profound impact on how military and civilian consumers will power portable devices, since batteries simply cannot match demands for runtime, energy density and reliability. A viable hydrogen storage system will enable direct hydrogen fuel cell based power systems, which are considered to be the best alternative to compete with lithium ion battery technologies, for portable applications.
In order to be competitive with lithium ion battery technology, a hydrogen storage cartridge must achieve volumetric energy densities on the order of 1200 Wh/L. The amount of fuel required in the hydrogen storage cartridge such that 1200 Wh/L are provided depends on the fuel material density and gravimetric hydrogen density. Because hydrogen has poor energy content per volume (0.01 kJ/L at standard temperature and pressure (STP) and 8.4 MJ/L for liquid hydrogen vs. 32 MJ/L for petroleum), physical transport and storage as a gas or liquid is impractical. Additionally, the compression process to achieve the pressures necessary to reach a high density is energy-intensive and doesn't solve the hazard issue. Also, the densities of compressed H2 are still below those required to reach practical fuel storage goals.
Physical means to store hydrogen include complex hydrides such as metal alanates, amides, and borohydrides. Complex hydrides are salts consisting of a metal cation and an anion containing hydride such as tetrahydroborate [BH4]− and tetrahydroaluminate [AlH4]−. The hydrogen in complex hydrides can be released hydrolytically, which is essentially an irreversible process, and by heating or reduction of vessel pressure, which are reversible processes if a pressure increase or temperature decrease also leads to readsorption of hydrogen. For hydrogen storage, Group 1 and 2 salts of alanates [AlH4]−, amides [NH2]− and borohydrides [BH4]− have been studied extensively and have high hydrogen gravimetric densities (Jain, et. al., J. Alloys & Compounds 2010, 03, 303-339; Orimo, et. al., (“Orimo”) Chem. Rev. 2007, 107, 4111-4132; Schüth, F. Eur. Phys. J. Special Topics 2009 176, 155-166; Eberle, et. al., Angew. Chem. Int. Ed. 2009, 48, 6608-6630.). However, high thermal stabilities and kinetics barriers to dehydrogenation and/or rehydrogenation in the solid state have thus far prevented complex hydrides from becoming practical hydrogen storage solutions.
Several methods have been investigated in an attempt to improve the thermodynamic properties of complex hydrides, such as formation of nanocomposites and through nanocatalysis (Orimo; Chen, et. al., Materials Today 2008, 11(12), 36-43.). The composite of lithium borohydride with magnesium hydride showed reduced enthalpy of decomposition from −67 to −42 kJ mol−1 with reversibility (Newhouse, et. al., J. Phys. Chem. C 2010, 114, 5224-5232; Vajo, Jet. al., J. Phys. Chem. B 2005, 109, 3719-3722.; Bösenberg, et. al., Acta Mater. 2007, 55, 39513958.). Another approach, doping or catalyzing a complex hydride with another material, was demonstrated using sodium alanate (NaA1H4) and select titanium compounds. Doped sodium alanate was shown to release hydrogen reversibly at temperatures as low as 100° C. Complex hydrides are an emerging and promising field of research for hydrogen storage. However, thermodynamics and kinetics remain challenging issues.
A recent approach for thermodynamic tuning is the investigation of alkali and alkaline earth metal mixed cation alanates and borohydrides (Graetz, J. Chem. Soc. Rev. 2009, 38, 73-82.). These types of compounds could include new compositions (inorganic ate complexes), reactive hydride composites (i.e., react during heating in the solid state and release hydrogen at temperatures below that of the constituent borohydrides), or react on heating as a simple mixture of constituent borohydrides (in other words, not be useful for the production of hydrogen gas). Mixed metal borohydride materials have been studied by many research groups in the past five to ten years.
The synthetic methods used to prepare these materials can be divided into three groups: solution synthesis, solid-state mechanochemical synthesis such as ball-milling, and high temperature/pressure synthesis (Hagemann, H.; Ĉerny, R. Dalton Trans. 2010, 39, 6006-6012.). The most prevalent method currently being used for the synthesis of the reported mixed metal borohydride ate complexes is ball-milling. However, a problem with mechanochemical synthesis, particularly for the synthesis of highly reactive materials, is the difficulty preparing pure, halogen-free samples of the expected products. Purification after mechanochemical synthesis is labor-intensive and can be difficult. High temperature/pressure synthesis is neither convenient nor scalable in a practical sense, since material impurities can have a profound impact on dehydrogenation temperature and material stability.
The objectives of the present invention include providing new mixed metal borohydride complexes that are suitable for solid hydrogen storage at practical operating conditions. The complexes are preferably stable under ambient conditions, including ambient air and temperature conditions. The objectives include complexes that can release hydrogen within a temperature range of 100° C. to 400° C. and more preferably within a range of 100° C. to 250° C. The objectives also include having a hydrogen storage material that has shelf stability. The objectives also include creating complexes with lower reaction temperatures and/or greater hydrogen storage capacities than any of the parent compounds.
According to another objective of the invention, there is provided a hydrogen generating apparatus that can be in any number of different embodiments. For example, the hydrogen generating apparatus can include a housing; a pellet strip including a flexible carrier and a plurality of pellets disposed on the carrier, each pellet including a mixed metal borohydride that will release hydrogen gas when heated; an ignition system comprising a heater; and a feed system configured to feed the pellet strip to sequentially position one or more pellets in proximity to the heater such that the heater is capable of heating the proximal pellet to release hydrogen gas. Embodiments can include one or more of the following features:                the pellet strip is wound on a reel disposed within the housing;        the hydrogen generator includes a plurality of pellet strips; the plurality of pellet strips can be disposed on a single reel, or at least one pellet strip can be disposed on each of a plurality of reels;        the pellet strip is in a folded configuration, preferably in a Z-fold pattern;        the pellets disposed on one section of the carrier are nested between the pellets disposed on another section of the carrier;        the hydrogen generator includes a plurality of pellet strips;        the carrier is in the form of a strip with surfaces on opposite sides thereof; the pellets can be disposed on one of the surfaces of the carrier, or the pellets can be disposed on both surfaces of the carrier; the pellets can be disposed in a linear array along the carrier; the pellets can be disposed in a plurality of linear arrays along the carrier;        the pellet strip is disposed in a storage compartment within the housing;        the hydrogen generator comprises a plurality of storage compartments within the housing, each configured to contain at least one pellet strip; each compartment can have a feed system configured to feed the at least one pellet strip therein;        the storage compartment is defined by a moveable wall; the moveable wall can be moveable to reduce the size of the storage compartment as the carrier and pellets are fed by the feed system; the moveable wall can separate the storage compartment from a waste compartment within the housing; a portion of the feed system can be moveable together with the moveable wall;        the feed system includes a sprocket that cooperates with the pellets disposed on the carrier; the sprocket can be an indexing sprocket; the feed system can include a ratchet configured to allow the carrier to be advanced in only one direction; the feed system can include a bellows that engages an escapement to rotate the sprocket;        the ignition system includes more than one heater;        the pellet strip is contained in a user-replaceable container;        each pellet includes at least one mixed metal borohydride; and        the pellet includes an ignition material.        
In another objective of the invention, there is provided another embodiment of a hydrogen generator. The hydrogen generator includes a sealable housing having an openable member and a hydrogen outlet; a plurality of fuel units removably disposed within the housing, each fuel unit including a casing containing a stack of pellets, and each pellet containing at least one mixed metal borohydride capable of producing hydrogen gas when heated; and a heating system for selectively heating one or more pellets to produce hydrogen gas. The heating system includes a plurality of heating elements disposed on an inside surface of the housing, each heating element in contact with a portion of the fuel unit casing that is in contact with one or more of the pellets contained therein. Heat selectively produced by one of the heating elements can be conducted through the corresponding portion of the fuel unit casing to heat one or more of the pellets. Embodiments can include one or more of the following features:                the heating system can further include a plurality of heat concentrators, each heat concentrator in contact with and configured to conduct heat to, a portion of a pellet; the heat concentrators can be disposed on external surfaces of the pellets; the heat concentrators can be at least partially disposed within the pellets; a single heat concentrator can be in contact with a single pellet; a single heat concentrator can be in contact with more than one pellet; more than one heat concentrator can be in contact with a single pellet;        the fuel unit includes thermal insulation between adjacent pellets;        each fuel unit casing includes a metal such as aluminum or stainless steel;        each fuel unit includes a hydrogen exit; the hydrogen generator can include one or more hydrogen flow paths from the hydrogen exits to the hydrogen outlet; one or more filters can be disposed within a portion of the one or more hydrogen flow paths;        the housing includes thermal insulation; and        the hydrogen generator includes at least a portion of a control system for controlling the selective heating of the pellets; the control system can be configured to monitor at least one of temperature and pressure and selectively heat one or more pellets based at least in part on at least one of the temperature and pressure.        
In another objective of the invention, there is provided another embodiment of a hydrogen generator, the hydrogen generator including a cartridge, a compartment configured to removably contain the cartridge, and an ignition system. The cartridge includes a sealed casing with a side wall, a base wall, and a lid; a plurality of pellets, each comprising at least one mixed metal borohydride capable of producing hydrogen gas when heated; a heat concentrator in direct contact with the casing and capable of conducting heat from the casing to the at least one reactant; a hydrogen outlet valve in the casing; and a hydrogen flow path from each fuel pellet to the hydrogen outlet valve. The compartment includes a housing with a side wall and a lid; a hydrogen outlet through the housing; a cavity within the housing within which the cartridge can be disposed; and a plurality of heating elements disposed on an inside surface of the housing, such that each heating element is in contact with an outer surface of the cartridge casing and aligned with a heat concentrator when the cartridge is disposed within the cavity. The ignition system includes the heat concentrators, the heating elements, and circuitry for conducting an electric current to the heating elements, such that the electrical current can be applied selectively to one or more heating elements for generating heat to selectively heat one or more pellets to initiate a reaction to produce hydrogen gas. Embodiments can include one or more of the following features:                the cartridge has a cylindrical shape;        the cartridge has a prismatic shape;        the cartridge and the compartment cooperate such that the cartridge can be inserted into the compartment only such that the heating elements and the heat concentrators are properly aligned;        the pellets are arranged in multiple layers, each layer having a single pellet;        the pellets are arranged in multiple layers, each layer containing more than one pellet;        the pellets are arranged in a single layer;        the heat concentrators are disposed on pellet surfaces;        the heat concentrators are partially disposed within the pellets;        each heat concentrator has a cartridge casing contact portion that extends beyond the pellet;        each heat concentrator comprise aluminum;        each heat concentrator comprises a layer of pyrolictic carbon in contact with the pellet;        the heat concentrators are in pressure contact with the inside surface of the casing;        a thermally insulating material is disposed between at least portions of adjacent pellets; a layer of the insulating material can separate layers of the pellets; pellet surfaces can be coated with a layer of the insulating material;        the portion of the cartridge casing that makes contact with the heating element comprises stainless steel or aluminum;        the housing comprises a material with low electrical and thermal conductivity;        the heating elements are disposed on an inside surface of at least one of the side wall, the lid and the door of the housing;        the heating elements make pressure contact with the outer surface of the cartridge casing when the cartridge is disposed within the compartment;        the cartridge can include means for maintaining contact between the heat concentrators and the solid compositions and/or maintaining alignment between the heat concentrators and the heating elements as the heat generator is being used;        the hydrogen flow path comprises a channel extending through all layers of pellets; the hydrogen flow path can include a central channel; the hydrogen flow path can comprise more than one channel;        at least one filter is disposed in the hydrogen flow path;        the cartridge comprises a foil seal over the hydrogen outlet valve prior to insertion of the cartridge into the compartment; the foil seal can be broken upon insertion of the cartridge into the compartment; and        the pellets further comprise an ignition material, preferably at least one material selected from the group of iron powder plus KC1O4, TiH2 plus KC1O4, Mn02 plus LiA1H4, Ni plus Al, Zr plus PbCrO4, Fe2O3 plus Al, and LiA1H4 plus NH4C1.        
In another objective of the invention, there is provided another embodiment of a hydrogen generator including a cartridge including a sealed casing and a plurality of pellets stacked within the casing, each pellet containing at least one mixed metal borohydride capable of producing hydrogen gas when heated; a compartment including a housing, a hydrogen outlet through the housing, and a cavity within the housing within which the cartridge can be removably disposed; and an induction heating system. The induction heating system includes a plurality of secondary coils within the cartridge casing, with each secondary coil in contact with one or more of the pellets. The induction heating system also includes at least one primary coil within the compartment housing. The induction heating system is configured to receive an electric current from a power source, provide an electromagnetic field from a flow of the current in the at least one primary coil, induce an electric current in the at least one secondary coil, and provide heat from a flow of the induced electric current; thereby heating the pellets. Embodiments can include one or more of the following features:                the cartridge has a cylindrical shape;        the cartridge has a prismatic shape;        the pellets are arranged in multiple layers, each layer having a single pellet;        each pellet is in contact with a secondary coil; each pellet can be in contact with a separate secondary coil; more than one pellet can be in contact with the same secondary coil;        the at least one secondary coil is in contact with a pellet surface;        the at least one secondary coil is disposed at least partially within a pellet;        the induction heating system includes a moveable primary coil;        the induction heating system includes a plurality of primary coils; the primary coils can be adjacent to, on or at least partially recessed in an inner surface of the compartment housing;        a thermally insulating material is disposed between adjacent pellets;        the cartridge can include means for maintaining contact between the solid compositions and the secondary coils;        the cartridge can include means for maintaining alignment between the secondary coils and the primary coils as the heat generator is being used;        the hydrogen flow path comprises a channel extending through and/or around the pellets; the hydrogen flow path can include a central channel; the hydrogen flow path can comprise more than one channel;        at least one filter is disposed in the hydrogen flow path;        the cartridge comprises a foil seal over the hydrogen outlet valve prior to insertion of the cartridge into the compartment; the foil seal can be broken upon insertion of the cartridge into the compartment; and        the pellets further comprise an ignition material, preferably at least one material selected from the group of iron powder plus KC1O4, TiH2 plus KC1O4, MnO2 plus LiA1H4, Ni plus Al, Zr plus PbCrO4, Fe2O3 plus Al, and LiA1H4 plus NH4C1.        
In yet another objective of the invention, there is provided a fuel cell system including a fuel cell with an anode, a cathode, and an electrolyte that that is used with any of the hydrogen generators as described herein. Hydrogen gas from a hydrogen generator is supplied to the anode of the fuel cell. Embodiments can include one or more of the following features:                the fuel cell cathode is supplied with oxygen.        the fuel cell electrolyte separates the anode and the cathode.        the fuel cell system further includes a control system configured to control an ignition system and a feed system based on at least one of a pressure within the fuel cell system, an electrical characteristic of the fuel cell system, or an electrical characteristic of an electronic device in electrical communication with the fuel cell system; the control system can include at least one of a microprocessor, a micro controller; digital circuitry, analog circuitry, hybrid digital and analog circuitry; a switching device; a capacitor, and sensing instrumentation.        the fuel cells are “stacked,” or placed in series or parallel circuits, to increase the voltage and current output to meet an application's power generation requirements.        the fuel cell system includes a control system for controlling the selective heating of the pellets; a portion of the control system can be disposed within the hydrogen generator; a portion of the control system can be disposed outside the hydrogen generator.        the ignition system is configured to monitor at least one of temperature and pressure and selectively heat one or more pellets to provide hydrogen as needed by the fuel cell stack.        a portion of the ignition system is in the fuel cell system outside the hydrogen generator.        
These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims, and appended drawings.